The lithium-ion battery cell is the premiere high-energy rechargeable energy storage technology of the present day. Unfortunately, its high performance still falls short of energy density goals in applications ranging from telecommunications to biomedical. Although a number of factors within the cell contribute to this performance parameter, the most crucial ones relate to how much energy can be stored in the electrode materials of the cell.
During the course of development of rechargeable electrochemical cells, such as lithium (Li) and lithium-ion battery cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Among these, occlusion and intercalation materials, such as carbonaceous compounds, layered transition metal oxides, and three dimensional pathway spinels, have proved to be particularly well suited to such positive electrode applications. However, even while performing reasonably well in recycling electrical storage systems of significant capacity, many of these materials exhibit detrimental properties, such as marginal environmental compatibility and safety, which detract from the ultimate acceptability of the batteries. However, of most importance is the fact that the present state of the art materials only have the capability to store relatively low capacity of charge per weight or volume of material (e.g. specific capacity, (mAh/g); gravimetric energy density (Wh/kg−1); volumetric energy density, (Wh/L−1)). Volumetric capacity is of particular importance in many applications.
Materials of choice in the fabrication of rechargeable battery cells, particularly highly desirable and broadly implemented Li-ion cells, for some considerable time have centered upon graphite negative electrode compositions, which provide respectable capacity levels in the range of 300 mAh/g. Unfortunately, complementary positive electrode materials in present cells are less effective layered intercalation compounds, such as LiCoO2, which generally provide capacities only in the range of 150 mAh/g. Other positive electrode materials of utility for primary lithium batteries include manganese oxides and silver vanadium oxides.
Intercalation compounds are not highly effective because the intercalation process is not an ideal energy storage mechanism. This situation occurs because of the limited number of vacancies available for the guest lithium ion. An alternative process reversible conversion, allows for all of the oxidation states of a compound to be utilized. The reversible conversion reaction proceeds as follows:zLi++ze−+MeXLizX+Mewhere Me is a metal and X is O2−, S2−, N3− or F−. This reaction can lead to much higher capacities than can an intercalation reaction and, therefore, to much higher energy densities.
Badway et al. (Journal of the Electrochemical Society, 150(9) A1209-A1218 (2003)), for example, has described electrode materials having high specific capacities via a reversible conversion reaction. They reported specific capacities for carbon metal fluoride nanocomposites, such as a carbon FeF3 nanocomposite, active for this reaction, having >90% recovery of its theoretical capacity (>600 mAh/g) in the 4.5-1.5 V region. They attained this major improvement in specific capacity by reducing the particle size of FeF3 to the nanodimension level in combination with highly conductive carbon.
Other metal fluorides are capable of reversible conversion as well. For example, bismuth metal fluorides, such as BiF3, are capable of reversible conversion. As mentioned above, reversible conversion reactions also have been observed in metal oxides as well as metal fluorides. Because metal fluorides are more ionic than metal oxides, the discharge voltage of a given fluoride compound will always be higher than the discharge voltage of the corresponding oxide, thereby leading to greater specific energies and attractiveness as future positive electrode materials. Another effect of the higher ionicity of the metal fluorides with respect to the metal oxides is that the generally lower band gap oxides have relatively good electronic conductivity while the high band gap fluorides are electronic insulators. As a direct consequence, it has been shown that the preparation of a metal fluoride/conductive matrix nanocomposite is necessary in order to enable the electrochemical activity of the higher voltage metal fluorides whereas the oxides can be utilized in their macro state.
The electroactivity of metal fluoride (e.g., BiF3, CuF2, and FeF3) conversion materials with relatively high output voltage (approximately 3V) and high volumetric energy density through the use of nanocomposites by introduction of highly conductive carbon black and/or mixed conductor matrices to the metal fluorides has been demonstrated (Badway, F., et al., J. Electrochem. Soc., 150, A1318, 2003; Bervas, M., et al., Electrochem. Solid-State Lett., 8, A179, 2005; Badway, F., et al., Chem. Mater., 19, 4129, 2007). Silver fluoride has a higher output voltage (>3.5V), energy density (9.1 Wh/cc) and an assumed high electrical conductivity with the formation of the Ag product of conversion than CuF2 (8.25 Wh/cc) and CF1 (6.79 Wh/cc) making it superior to CuF2 and CuF1. However, experimentation has shown the highly oxidative AgF2 excludes the conventional approach to enable its electroactivity through nanocomposites by carbon black.
Hence, there is a need in the art for electrical energy-storage and delivery capable of using stabilized silver fluoride within nanocomposites or new structures in order to combine both good electronic conductivity as well as high voltage capabilities.